Metal–Ligand Cooperation in N–H Activation: Bridging Electron-Pushing Formalism and Energy Descriptors

The activation of
N–H bonds is a fundamental step
in the
synthesis of industrially relevant compounds but remains a challenging
process. A promising strategy to address it, introduced by Milstein
and co-workers, relies on metal–ligand cooperation, in which
N–H activation is coupled with an aromatization–dearomatization
process of a pincer ligand. In this work, we employ
state-of-the-art theoretical methods grounded in quantum chemical
topology (QCT) to gain deeper insights into this process. Using the
archetypal PNP–Ru(II) complex reported by Milstein (JACS 2010,
132, 8542), we analyze the electron density rearrangements during
N–H activation through the electron localization function and
bonding evolution theory. Interacting quantum atoms energy decomposition
is further applied to quantify interactions between key groups. The
study covers substrates from ammonia to primary amines, revealing
that hydrogen transfer occurs as a quasi-protonic species, yielding
a Ru–amido complex. The mechanism remains consistent across
substrates, with electron-withdrawing groups facilitating the process
by stabilizing the NH–R interaction. Additionally, modifying
the ligand scaffold with electron-donating substituents enhances charge
accumulation at the reactive carbon, improving both kinetics and thermodynamics.
Overall, our findings highlight QCT as a powerful framework for guiding
the rational design of improved systems.